A computer system that has to be on all the time may have more than one hot-pluggable power supply to supply electrical power to the components of the computer system. Under normal operating conditions, the power supplies work together to supply the electrical power for the computer system. Thus, each power supply generates its share (equally) of the total power required by the computer system. When one of the power supplies fails or is removed from the computer system, then the remaining power supply (or supplies) generates the entire amount of the power for the computer system. When a replacement power supply is inserted into the computer system, each power supply again generates its share (equally) of the total power requirements. In this manner, the computer system is ensured to operate almost all the time, even if one of the power supplies should fail or be replaced.
During normal operation, each power supply generates its share of the required electrical power at the same voltage level as the other power supply and supplies the electrical power through a common system power bus. Upon failure or removal of one of the power supplies, the remaining power supply quickly increases its power output to compensate for the loss of the other power supply. On the other hand, upon insertion and activation of a second power supply while a first power supply is operational in the computer system, the first power supply quickly decreases its power output while the second power supply is activated and increases its power output. Before the second power supply can become fully operational, however, reverse electrical current may flow from the power bus into the output of the second power supply. The reverse current may cause voltage fluctuations on the system power bus and in-turn disrupt the operation of the computer, unless proper precautions are taken in the design of the power supplies.
A diode, or a MOSFET configured to operate as a diode, is commonly placed on the output of the power supplies to prevent the current from flowing into the inserted or failed power supply. The diodes and MOSFETs used in this manner are commonly referred to as “ORing” diodes and MOSFETs. The ORing diode or MOSFET is reversed biased in this situation and does not allow the reverse current to flow. Under normal (i.e. forward biased) operation of the power supply, however, the ORing diode dissipates a portion of the power produced by the power supply, thereby negatively affecting the efficiency of the power supply. The ORing MOSFET dissipates less power than the ORing diode, but increasing numbers of such ORing MOSFETs are required as the total amount of power produced by the power supplies is increased, thereby increasing the number of components for (and the cost and size of) the power supplies.